1. Field of the Invention
The present invention relates to a foil shield for use with a vacuum pump having a high speed rotor. The invention also relates to a vacuum pump with a foil shield.
2. Description of the Prior Art
Vacuum pumps with high-speed rotors are successfully used for generation of high and ultra-high vacuum. One of the most common types of vacuum pumps, which are used for generation of high and ultra-high vacuum, is turbomolecular pumps which are also called turbo pumps. In these pumps, rotor and stator discs, which are provided with blades, are arranged alternatively with each other, with the rotor having a rotation frequency between 10 s−1 and 1000 s−1 (revolutions per second). When these vacuum pumps are used in environments with high magnetic fields, eddy currents are generated in the rotor. The eddy currents lead to heating of the rotor and, thereby, to its linear extension. With a necessary small gap width of the vacuum pumps, the linear extension is critical and can result in a contact of the rotor with the stator. On the other hand, the eddy currents cause braking of the rotor and associated therewith a high power consumption of the drive.
Opposite is also problematic. The high-speed rotors are often magnetically supported, i.e., the rotor is supported by magnetic forces, without any mechanical contact. A magnetic field which is generated in a magnetic bearing is not limited to the bearing space during its spatial expansion. The magnetic field lines can emerge out of the suction opening of the pump and cause disturbances in apparatuses located in front of the suction opening. One of such apparatuses can be an electronic microscope in which the stray field of the magnetic bearing can lead to deflection of an electron ray and, thereby, to loss or reduction of its resolution. Because of ever greater sensitivity and the required better resolution, this ray deflection can be tolerated in a very small amount.
According to the state of the art, this problem is attempted to be solved by shielding the pump housing and the rotor shaft (z. Vakuum—Technik, 27, vol. 1, pp. 6-8, Vacuum-Technology).
Another solution is proposed in German patent number 3,531,942. The proposed solution lies in suppressing of eddy currents, with the rotor and its components being formed of a material with a specific resistance of 10−4 Ωm or more. A particularly recommended material is silicon nitride.
The solutions according to the state of the art have many drawbacks. The shielding of the housing produces unsatisfactory results. Additional shielding of the rotor shaft is technically difficult from the manufacturing point of view. The proposed material selection is extremely expensive and is not suitable for wide use with a large number of produced items.
Accordingly, an object of the invention is a significantly improved magnetic decoupling of the interior of a pump from its surrounding, without using expensive measures, so that solution remains cost-effective.